Direct conversion receivers (a.k.a. homodyne or zero-IF receivers) and heterodyne receivers (a.k.a. low-IF receivers) are commonly used today to receive quadrature modulated signals sent over a communications network. The types of networks that use such receivers include satellite communication networks, wireless cellular networks, local access networks (LANs), wide access networks (WANs), as well as other types of communication networks. FIG. 1 is a simplified overview of a satellite communications network 100. The satellite communications network of FIG. 1 is an example, but it will be understood by those skilled in the art that the following discussion applies equally to other types of communication networks.
A gateway terminal (commonly referred to simply as a “gateway”) 102 sends and receives signals to a satellite 104 through a satellite antenna 106. Signals transmitted from the gateway 102 to the satellite 104 are commonly referred to as forward uplink signals 108. Signals that are received by the gateway 102 from the satellite 104 are commonly referred to as return downlink signals 110.
The satellite 104 receives the forward uplink signals 108 sent from the gateway 102 and retransmits them to one or more user terminals 112 within the satellite communications network 100. Signals transmitted from the satellite to a user terminal 112 are commonly referred to as forward downlink signals 118. Signals received by the satellite from the user terminal 112 are commonly referred to as return uplink signals 120. Two user terminals 112 are shown. However, a typical satellite communications network 100 comprises many such user terminals. The user terminals 112 may be either fixed in their location or mobile terminals. Each user terminal 112 may have an antenna 122 and may comprise multiple elements including, for example, an outdoor unit (not shown), an indoor unit (not shown), and a link (not shown) there between.
In some cases, the gateway 102 communicates over a “backhaul” 116 with a terrestrial communications network 114, such as the Internet to allow the satellite 104 to connect a user terminal 112 to the terrestrial communications network 114.
FIG. 2 is a schematic of a portion of a direct conversion receiver, such as might be found in a user terminal 112 shown in FIG. 1. (As noted, a user terminal 112 can alternatively have a heterodyne receiver.) A composite quadrature receive signal 202 is applied to the data input of both an I-mixer 204 and a Q-mixer 206. A receiver local oscillator (LO) 208 is coupled to a phase splitter 210 that has an I-output that is at a reference phase (commonly referred to as 0 degrees) and a Q-output at the same frequency, but shifted 90 degrees with respect to the I-output. The I-output is coupled to the LO input of the I-mixer 204. The Q-output is coupled to the LO input of the Q-mixer 206. The outputs 212, 214 of the mixers 204, 206 are then digitized in analog-to-digital converters (ADC) 216, 218.
One problem that occurs with direct conversion receivers and to a certain extent with heterodyne receivers is that there can be an imbalance between the in-phase leg and the quadrature leg of either the transmitter from which the quadrature modulated signal is sent or the receiver that receives the signal. For example, imbalances in the amplitude or the phase of the in-phase signal 212 output by the I-mixer and quadrature signal 214 output by the Q-mixer can make it difficult to discriminate between symbols. That is, symbols are distinguished from one another based on the relative amplitude of the in-phase component 212 and the quadrature component 214 of the received signal 202. Therefore, differences in the gain and phase of the LOs or the path through the ADCs 216, 218 can lead to errors in identifying the symbol represented by the combination of in-phase and quadrature components of the received signal. Additionally, differences in the mixers 204, 206 and inaccuracy of the 90 degree phase splitter 210 can all result in an I/Q imbalance error. In addition, due to several causes that are well known in the art, it is possible for the output of the mixers 204, 206 to have a direct current (DC) offset. Such a DC offset can cause, among other problems, the ADCs 216, 218 to saturate prematurely.